1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of the Related Art
There is disclosed in Japanese Patents No. 2727558 and 2805781 a spark plug for an internal combustion engine wherein carbon fouling (contamination) on an insulator tip is burned down by sparks generated during a spark discharge in a discharge gap.
Concretely, an atmosphere around electrodes is ionized by a leak current flowing through the carbon fouling from a time when a high voltage is applied to the electrodes to a time when a capacitive discharge is caused at tips of the electrodes. Then, when an inductive discharge is caused following the capacitive discharge, the carbon fouling on the insulator around the central electrode is burned down, thereby recovering an insulating resistance.
The central electrode of the spark plug is provided with a narrowed portion (narrower than a body) extending to the earth electrode. The ionization by the above-mentioned leak current is promoted by disposing a border portion between the body and the narrowed portion in the insulator.
Although the carbon fouling is burned down at a certain degree according to the above-cited Japanese Patents, it is being desired to further completely remove the carbon fouling, due to demands for further improvements of starting capability under a cold weather and of a drivability.